[FSR 51] For NASA spaceflight structures made of well-characterized materials and with sufficient load cycle data that accounts for all in-service environments, a minimum service life factor of 4.0 shall be applied to the service life for fatigue and creep life assessments.
For structures made of materials that are not well characterized or those that may have complex failure modes such as composite structures, an additional factor and testing may be required by the delegated Technical Authority.